Field of the Invention
The present invention relates generally to a method of fabricating an electronic circuit device, and, more particularly, to a method of soldering leads of circuit elements, face-mounted circuit elements, and the like on an electronic circuit board in a fluxless manner.
Conventionally, soldering circuit elements to an electronic circuit board typically requires flux to enhance the flow of the solder as the circuit element is being mounted to the board. However, the flux leaves a residue after the soldering operation is completed, which residue must be removed by cleaning. A fluxless solder paste has been developed, which enables a soldering operation to be performed without subsequent cleaning (see, for example, Japanese Patent Laid-Open Nos. Hei 2-25291 and Hei 2-290693).
In Japanese Patent Laid-Open No. Hei 2-25291, a solder paste used in a reducing atmosphere is disclosed, which comprises a solder powder and a binder containing an alcohol. The method does not generate a residue because the solder paste contains no solid component such as an active agent or a resin. Moreover, the boiling point of the alcohol contained in the binder lies within a boundary region (paste region) between a solidus line and a liquidus line in a phase diagram of the solder powder. Thus, the alcohol boils and evaporates when the solder arrives at a semimolten state when heated. Consequently, the method also solves a prior problem suffered by a solder paste containing an alcohol having a boiling point below the solidus line, wherein solder balls are scattered upon evaporation of the alcohol.
Nevertheless, this method exposes the solder surface to the atmosphere when the solder is melted for bonding, upon evaporation of the alcohol. Thus, a reducing atmosphere (such as hydrogen gas) is required for the solder operation, to prevent oxidation of the solder. Moreover, the method has a limited reliability for high-density packaging because the solder paste is typically supplied to a bonding portion by a coating technique such as printing, which has a limited accuracy.
In Japanese Patent Laid-Open No. Hei 2-290693, a solder paste is disclosed for use in an inert atmosphere. The solder paste comprises a fine solder powder and an alcohol having a boiling point higher than the melting point of the solder. Thus, the alcohol is present throughout the soldering process. In other words, the bonding portion is shielded from the atmosphere by the alcohol upon soldering, so that the soldering can be carried out in the inert atmosphere. Consequently, the method is advantageous in eliminating the necessity of a special atmosphere such as the reducing atmosphere required by the method of JP 2-25291. However, the method of JP 2-290693 is also typically employed in a coating technique such as printing, and thus has a reliability that is limited by the accuracy of the printing technique.